DEPARTMENT OF COMMERCE 


Circular 

of THE 

Bureau of Standards 

S. W. STRATTON, Director 


No. 108 

GYPSUM—PROPERTIES, DEFINITIONS 

AND USES 


JANUARY 3, 1921 



PRICE, S CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 

Washington, D. C. 


WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1921 











DEPARTMENT OF COMMERCE 


Circular 

of THE 

« Bureau of Standards 

S. W. STRATTON. Director 


No. 108 

GYPSUM—PROPERTIES, DEFINITIONS 

AND USES 


JANUARY 3, 1921 



PRICE, 5 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 

Washington, D. C. 


WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1921 



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LIBRARY OF CONGRESS 

"ECSIYED 

FEB 4-1921 

DOCUMENTS DIVISION 



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GYPSUM—PROPERTIES, DEFINITIONS, AND USES 


ABSTRACT 

This paper contains brief descriptions of the method of manufacture, properties, 
and uses of the various products made from gypsum. 

Gypsum is a rock which consists of calcium sulphate combined with water. It 
is used in the raw state for the manufacture of Portland cement and as a fertilizer. 
When heated to such an extent that three-fourths of the water is driven off, the 
product is known as calcined gypsum or plaster of Paris, which has a great variety 
of uses. The addition of retarder, hydrated lime, fiber, and sometimes sand, to 
calcined gypsum makes the gypsum wall plaster of commerce. From the stand¬ 
point of tonnage this is the most important gypsum product. When raw gypsum is 
heated so that all of the water is driven off, the product sets too quickly to be market¬ 
able, but is used at the factory to make such products as gypsum tile, gypsum 
plaster board, gypsum wall board, etc. This is the best material to use for pouring 
gypsum reinforced structural shapes, cast in place, although calcined gypsum can 
be used for this purpose. 

The Gypsum Industries Association has established a fellowship at this Bureau to 
assist in developing information about the properties and uses of the various products. 

Specifications for calcined gypsum, neat gypsum plaster, gypsum plaster board, 
and gypsum wall board are included in this paper. 


CONTENTS 

Page 


1. Introduction. 4 

2. Gypsum. 4 

(a) Occurrence and properties. 4 

( b ) Commercial forms and uses.-. 5 

3. Calcined gypsum. 5 

(a) Manufacture. 6 

( b ) Properties. 6 

(c) Commercial forms and uses. 7 

4. Hardening of calcined gypsum. 8 

5. Gypsum wall plasters. 9 

6. Second-settle calcined gypsum. 10 

7. Further products of calcination. 11 

(a) Keene’s cement. 11 

8. Structural gypsum products. 12 

(a) Precast products. 12 

(1) Gypsum tile. 12 

(2) Gypsum plaster board. 12 

(3) Gypsum wall board. 13 

( b ) Structural members cast in place. . :. 13 

9. Packages and storage. 14 

10. W T ork of the Bureau of Standards. 14 

11. Specification for calcined gypsum. 15 

12. Specification for neat gypsum plaster. 17 

13. Specification for plaster board. 17 

14. Specification for gypsum wall board. 19 


3 





























4 Circular of the Bureau of Standards 

1. INTRODUCTION 

The present generation has seen the rapid development of the 
gypsum industry to a place of permanence and importance in the 
building trade. Plaster of Paris and hard wall plaster are gypsum 
products with which the public is quite familiar, and many new 
uses for gypsum products, other than as a building material, have 
been discovered. Gypsum has several valuable properties which 
are not possessed by any other material. It is therefore adapted 
to a large variety of uses, not all of which have as yet been com¬ 
mercially developed. In order to take advantage of this oppor¬ 
tunity for the expansion of the industry, it is well first to have a 
knowledge of the different kinds of gypsum products now on the 
market. Particularly should their names be studied, for the 
rapid growth of the industry has brought about a confusion of 
trade terms, which very possibly affects the consumption of the 
materials. There is a natural reticence about buying or using a 
material the name of which is not thoroughly established and 
understood. An explanation of the nomenclature in present use 
may have the further function of helping to establish definite 
meanings for the terms. 

2. GYPSUM 

(a) OCCURRENCE AND PROPERTIES 

Gypsum is a rock, which usually occurs bedded in the ground 
in the same way as limestone or any other of our sedimentary 
rocks. Pure gypsum is pure white in color and has a translucent 
sheen. It is apt, however, to be colored by small amounts of 
certain impurities, particularly iron. Its most distinguishing 
characteristic is its extreme softness, for it can be readily scratched 
with a pen knife. Not all gypsum is bedded. Sometimes it 
occurs in pockets or even as bowlders distributed through a magma 
of clay or limestone. A variety of gypsum, which may be re¬ 
garded as highly consolidated, so that the crystals have disap¬ 
peared and the mass has become translucent, is known as alabaster. 

• Gypsite is another variety, which has not been consolidated, but 
occurs in loose form, frequently mixed with considerable amounts 
of dirt. Gypsite, gypsum, and alabaster bear a relation to each 
other which is analogous to the relation between marl, limestone, 
and marble. 

Gypsum is a very common mineral. Targe deposits of it occur 
in many localities throughout the United States. It is mined 
chiefly in New York, Iowa, Michigan, Ohio, and Texas. Many 


Gypsum — Properties, Definitions, and Uses 5 

other States produce smaller amounts, and there are a number of 
known deposits which have not yet been commercially developed. 
A large part of the gypsum used on the Atlantic seaboard is mined 
in Nova Scotia. Gypsite is mined in Kansas and Oklahoma. 
Alabaster is not produced in the United States. 

Chemically, gypsum is a hydrated sulphate of calcium. The 
lime and the sulphuric acid are combined in exact equivalents, so 
that the product is neither acid nor basic, but neutral. The pure 
material consists, approximately, of 32.6 per cent lime, 46.5 per 
cent sulphuric anhydride, and 20.9 per cent water. Anhydrite is 
another material which is of similar, but not identical, composi¬ 
tion and which is frequently found associated with gypsum. It 
is simply calcium sulphate without any water. 

(6) COMMERCIAL FORMS AND USES 

While most gypsum is calcined before it is put on the market, 
about one-fifth of the total output is sold in the raw state, either 
crushed or ground. Practically all of this is used for one of two 
purposes—the manufacture of Portland cement or as a fertilizer. 

Freshly made Portland cement sets, or hardens, much too 
quickly to permit of its being used conveniently. It has been 
found that a small amount of gypsum will retard this setting. 
The gypsum, in an amount up to 3 per cent of the cement, is added 
just before the final grinding, so that it can be thoroughly mixed. 
Either raw or calcined gypsum will have the same effect, but the 
total tonnage of raw gypsum which is actually used for this pur¬ 
pose is about five times that of the calcined material. There is a 
possibility that anhydrite is equally suitable. It is furnished in 
any size from 3 inches down to dust, to suit the requirements of 
the purchaser. 

Finely ground raw gypsum has attained an established position 
as a fertilizer under the name of “land plaster.” The calcium 
and the sulphur are both necessary plant foods, and its compara¬ 
tively high solubility renders it quickly available. The purity 
of gypsum sold for this purpose is usually prescribed by State 
law. Commercial products are now to be had so fine that 90 per 
cent of the material will pass a No. 100 sieve. 

3. CALCINED GYPSUM 

Most gypsum is calcined before it is put on the market. The 
process of calcination may be defined as the heating of the gypsum 
under definite conditions to produce a definite material. 


6 


■ Circular of the Bureau of Standards 

(a) MANUFACTURE 

The raw gypsum is first crushed, then dried if necessary, and 
finally ground to a fine powder before it is fed into the calcining 
kettles. These kettles are aptly described by their name. They 
are made of iron, and come in various sizes, holding from 2 to 20 
tons. Each kettle is provided with a paddle which rotates on a 
vertical shaft and keeps the mass stirred up. The kettle is set 
in brickwork and heated by direct fire. A few rotary calciners are 
now in use. These are quite similar to the rotary kilns used in 
making Portland cement and lime. If a rotary calciner is used, 
the gypsum is crushed to about three-fourths inch before calcina¬ 
tion, but is not finely ground until afterwards; otherwise there 
would be too much dust carried up the stack. 

It was noted above that pure gypsum contains about 20.9 per 
cent water. This water is held in chemical combination, so that 
the material is normally dry, but the water can be driven off by 
heat. As the steam escapes from the kettle, it tends to carry the 
powdered gypsum up with it. The whole mass fluffs up and 
appears to boil. Eventually this violent action ceases, and the 
mass settles down in the kettle to about its original volume. The 
material in the kettle at this point is known as “calcined gypsum,” 
sometimes called “first-settle stucco.” 

( b ) PROPERTIES 

“Calcined gypsum,” then, is gypsum from which a part of the 
water has been removed by heat. Theoretically, this water 
is given off at ' 107° C. Practically a slightly higher tem¬ 
perature is used. It takes about an hour to calcine a kettle full 
of gypsum to “first settle.” The chemical reaction involved is: 
2(CaS0 4 .2H 2 0) +heat — 2CaS0 4 .H 2 0 + 3H3O. Calcined gypsum is 
therefore 2CaS0 4 .H 2 0, or, as it is more usually written, 
CaS 0 4 . 3 TH 2 0 . From this formula it is frequently called the 
“ hemi-hydrate.” Calcination reduces the water content from 
20.9 per cent to 6.2 per cent and the proportions of the lime and 
sulphuric anhydride are thereby raised to 38.6 per cent and 55.2 
per cent, respectively. 

Calcined gypsum has many peculiar and valuable properties. 
When mixed with water to form a plastic mass, it sets or hardens. 
This set can be easily regulated to take place in any desired 
time. The setting is accompanied by expansion, which property 
is of great value in making casts or molds. The set material is 
nearly as strong as Portland cement, and yet is soft enough so 


Gypsum — Properties, Definitions, and Uses 


7 


that it can be readily tooled. Its porosity and consequent ability 
to absorb water is important when it is used for making pottery 
molds. Its ability to set before it dries is the property which 
makes it valuable as a gaging plaster. 

(c) COMMERCIAL FORMS AND USES 

i 

Calcined gypsum is probably best known to the general public 
as “ plaster of Paris,” but it is sold under a great variety of names, 
depending upon the purpose for which it is to be used. Thus 
“dental plaster” is used by dentists and physicians for making 
plaster casts and impressions. “Molding plaster” is used for 
making the plaster decorations with which ceilings and walls are 
sometimes embellished. If the material is used in making plaster 
casts, statuary, etc., it is called “casting plaster.” Under the 
name of “potters’ plaster” it is used for making molds in which 
terra-cotta, porcelain vases, and other clay products are molded. 
It is used by the plasterer to mix with lime putty for the “white 
coat” of plaster, in which case it is called “gaging plaster.” All 
of these materials are essentially calcined gypsum, differing from 
each other chiefly in the extent to which the time of set has been 
regulated. 

It is unfortunate that such a confusion of nomenclature has 
come into common use. This is particularly true because there 
is a tendency now to acknowledge that all of the above names are 
incorrect. The word “stucco” has been defined by architects 
and builders to mean a material which is applied in a plastic 
condition to form that surface of a wall which is exposed to the 
weather, and which will harden or “set” on exposure. This 
definition is in contradistinction to the word “plaster,” which is 
the same kind of material, but which is used only on interior 
walls and ceilings. The material which the gypsum man calls 
stucco could never be used to make stucco in the way in which the 
architect understands the word. Nor is the material in any 
sense a plaster except possibly that small amount of it which 
is used as gaging plaster. The gypsum manufacturers have been 
broad enough to see this difficulty and to adopt severe means 
to meet it. They have decided to drop all of the names above, 
and to call this material simply “calcined gypsum.” Such action 
is worthy of commendation and support. Of course, such radical 
changes can not be brought about by legislation, so that it is 
hardly likely that this generation will be able to declare the old 
terms obsolete. 


8 


Circular of the Bureau of Standards 

Many of the minor uses of calcined gypsum have been desig¬ 
nated above in the paragraph on nomenclature. Next in com¬ 
mercial importance to its structural uses is its use as a retarder 
for Portland cement. For this purpose, however, much more 
gypsum is sold raw than calcined. In the manufacture of plate 
glass, the glass is bedded in gypsum to hold it while grinding the 
surface. Calsomine, or cold-water paint, is a mixture of calcined 
gypsum and specially prepared glue, with or without pigments. 

4. HARDENING OF CALCINED GYPSUM 

When calcined gypsum sets, it first dissolves in the mixing 
water. It then combines with the same amount of water that 
was driven off during the calcination process, and thereby forms 
a compound which is identical with raw gypsum rock. This raw 
gypsum, being less soluble than the calcined form, crystallizes out 
of solution, the crystals interlocking with each other and giving 
strength to the mass. Since it requires heat to calcine gypsum, 
it follows that the change back from the calcined to the original 
form is accompanied by an evolution of heat. 

The normal time of set is from 5 to 10 minutes; that is, the 
mass will become hard in 5 or 10 minutes after the calcined 
gypsum is mixed with the water. The final setting may be 
hastened by any means which will aid crystallization. Usually 
acceleration is accomplished by adding a few crystals of set 
gypsum to the material. These form nuclei for the crystallization 
of the rest of the mass, and thereby hasten the reaction. Dental 
plasters are frequently accelerated so that they will set in one or 
two minutes. Conversely, any material which tends to prevent 
crystallization will retard the final setting. It has been found 
that colloidal substances are very effective for this purpose. The 
retarder generally used is a by-product of the tanning industry, 
similar to glue. Some ready-mixed wall plasters are retarded to 
set in not less than 6 hours. 

Evidently, the time of set of calcined gypsum can easily be 
controlled to suit the purpose for which it is to be used. This is 
a distinctive property of this material, and a very valuable one. 
Accelerators and retarders apparently have little effect on the 
rate of setting; they merely change the time at which the reaction 
begins. Judging from the evolution of heat, the reaction in all 
cases seems to be complete in about half an hour from the time it 
begins. After the crystallization is complete, further time can 
not be expected to show further change. Maximum strength is 


Gypsum — Properties, Definitions, arid Uses 9 

developed immediately upon the completion of the set. Of course, 
freshly set gypsum contains a great deal of excess water, and, 
in common with all similar materials, it is weak on this account. 
Drying may be accomplished quickly, however, so that an object 
made of gypsum can be brought to its final strength and condition 
in a few hours after the calcined gypsum is mixed with water. 
It is particularly to be noted that this setting does not depend 
upon exposure; the mixing water furnishes the only ingredient 
necessary for its completion. 

The compressive strength of a good sample of set gypsum 
(when tested in the form of a cylinder 2 inches in diameter by 
4 inches high) is about 2000 pounds per square inch. Its ten¬ 
sile strength is about 400 pounds per square inch. The age of 
the specimen makes no difference, provided only that it has had 
time to set completely, and that it is dry. The quantity of water 
used for mixing the calcined gypsum does decidedly affect the 
strength of the set material. Use as little water as possible and 
still have the mixture workable. 

5. GYPSUM WALL PLASTERS 

About three-fourths of all the gypsum mined finds its final 
market as wall plaster. The simplest form of gypsum sold for 
this purpose is the straight calcined gypsum, which is used as an 
addition to lime putty for white-coat vvork. This is sold as “ gaging 
plaster,” sometimes called “stucco,” and can be had either unre¬ 
tarded or retarded to set in about 20 minutes. 

Calcined gypsum does not work so smoothly under the trowel 
as could be desired. As a concession to the plasterer, therefore, 
it has become common practice to add 10 per cent to 15 per cent 
of clay or hydrated lime to the material at the mill. The mixture 
so produced, retarded to set in from one to six hours, is the gyp¬ 
sum plaster of commerce, which is sometimes termed “hard wall 
plaster.”' It is also sold as “neat gypsum plaster” in the East 
and as “cement plaster” in the West. The adjectives “fibered” 
and “unfibered” denote whether or not any hair or wood fiber 
has been added to it. 

Neat gypsum plaster is designed to be used for the scratch and 
brown coats of plaster. For this purpose sand must be added 
to it on the job. For scratch-coat work it is customary to use 
1 part of neat plaster by weight to 2 parts of sand; for brown 
coat the proportions are 1 to 3. Plaster to which the sand has 
22849°—21 - 2 



io Circular of the Bureau of Standards 

been added at the mill is now obtainable. It is known as “gypsum 
sanded plaster,” “ready-mixed scratch coat,” or “ready-mixed 
brown coat.” 

Another variety is known as “wood-fiber plaster.” This is 
neat gypsum plaster containing about i per cent by weight of 
wood fiber. It is designed to be used with or without sand, 
depending upon the nature of the base or background to which 
it is to be applied. 

Wall plasters and other building materials made of gypsum 
have a remarkable ability to resist fire. 1 The reason for this is 
due to the chemically combined water of crystallization, which, 
as stated, is about 20 per cent by weight. The heat breaks up 
these crystals and liberates the water, the process being slower 
as the heat penetrates farther into the gypsum. As long as there 
are any water crystals in the gypsum to be broken up the ma¬ 
terial will not warm appreciably above the temperature of boil¬ 
ing water. 

6 . SECOND-SETTLE CALCINED GYPSUM 

If, instead of emptying the kettle when the material first sub¬ 
sides in it, the heating is continued, the former phenomena of 
ebullition and subsidence will be repeated. When the material 
has settled down the second time no further apparent effect can 
be obtained by further heating. This material is known as 
“second-settle calcined gypsum,” sometimes called “second- 
settle stucco.” 

The reaction involved in this second calcination is expressed 
by the equation 2 CaS 0 4 .II 2 0 + heat = 2 CaS 0 4 + H 2 0 . That is, 
second-settle stucco is anhydrous calcium sulphate, all of the 
water having been driven off by the calcination. This substance 
is identical in chemical composition with the naturally occurring 
mineral, anhydrite. But there is a vast difference in their physical 
and chemical properties. To indicate this distinction, second- 
settle stucco is frequently called “soluble anhydrite.” They both 
have the same crystal form but different optical properties. 
Both forms react with water, but at extremely different rates. 
It is a matter of weeks before natural anhydrite can be made to 
combine with water to iorm set gypsum. Soluble anhydrite 
seizes water with avidity. When mixed to a plastic condition it 
sets more rapidly than calcined gypsum (the hemi-hydrate). A 


1 Gypsum as a Fireproofing Material, by Virgil G. Marani, Journal of the Cleveland Engineering 
Society; November, 1914. 



Gypsum — Properties, Definitions, and Uses n 

few minutes exposure to the air suffices for it to absorb enough 
water to revert to the hemi-hydrate. 

Besides its quicker set, soluble anhydrite has other advantages 
over calcined gypsum. It requires less water to make it plastic, 
and consequently makes a harder, denser, stronger set material. 
Unfortunately, as noted above, it will not keep. Manufacturers 
tried to put soluble anhydrite on the market, but found that by 
the time it reached the consumer it had largely reverted to cal¬ 
cined gypsum, so that the extra expense caused by the second 
calcination was thrown away. 

Large quantities of second-settle calcined gypsum are made for 
immediate use at the mill in the manufacture of precast gypsum 
tile (plain or reinforced), plaster board, wall board, etc. 

7. FURTHER PRODUCTS OF CALCINATION 

The change from calcined gypsum to soluble anhydrite occurs 
at 250 to 300° C. If the heating is prolonged, soluble anhydrite 
gradually changes its physical structure and becomes natural 
anhydrite. This is complete just below a red heat. 

Natural anhydrite will set if given time enough. It is used 
in Europe under the name of “flooring gypsum.” It sets into a 
very dense, hard mass, which makes an admirable floor. The 
American public, however, has not time to wait for it to set, so 
that it has never been put on the market in this country. 

(a) KEENE’S CEMENT 

Keene’s cement is a gypsum product which is used for wall 
plaster. It sets more slowly than calcined gypsum, but produces 
a harder, denser surface. The original process for making this 
material was patented, and therefore a matter of public knowledge, 
but it is believed that new secret processes have completely 
replaced the original method. Judging from its composition and 
behavior, it would seem that Keene’s cement is natural anhydrite 
to which some accelerator such as alum or borax has been added. 
This material can also be produced by burning select, ground, 
white gypsum at high temperatures. 

Further heating of natural anhydrite to about 1300° C will 
eventually result in its decomposition. Gaseous sulphur trioxide 
is evolved, and lime remains. This reaction finds no commercial 
use at present. 


12 Circular of the Bureau of Standards 

8. STRUCTURAL GYPSUM PRODUCTS 
(a) PRECAST PRODUCTS 

(1) GYPSUM TILE 

The quick-setting property of gypsum and its high strength 
make it an ideal material to cast into tiles at the mill. These 
tiles are shipped as such, and laid up to form walls, in the same 
way as brick or stone. Gypsum tiles come in a great variety of 
sizes, to suit all purposes. One size commonly used for building 
partitions is 12 inches wide by 30 inches long by 3 inches thick. 
They are usually “cored”—that is, holes run through them longi¬ 
tudinally—although solid tiles can be obtained. These holes are 
introduced principally to make the tiles lighter, thereby reducing 
the freight costs and the labor of handling. The air spaces which 
they form may assist the tile somewhat to resist the transmission 
of heat. For the same reasons, the tiles are usually made of 
calcined gypsum in which is included not more than 5 per cent, 
by weight, of wood fiber. This makes them of such a nature that 
they can be cut with a wood saw. Gypsum tiles must be set in 
gypsum mortar, made of 1 part calcined gypsum, by volume, to 2 
parts of sand. One tile of the material and the dimensions 
noted above will weigh about 30 pounds. When laid on edge, 
the usual method, it can be expected to carry a load of 1260 
pounds with a large factor of safety. The surfaces of these tiles 
are usually roughened to form a key for the plaster. 

In the design of precast reinforced gypsum tile, strength rather 
than lightness is the paramount consideration. These tiles do 
not contain any sand or fiber, and are of solid or cored pattern.^ 
They are made of second-settle calcined gypsum, with as little 
water as possible, so as to develop maximum strength. Tiles of 
this character are used mainly for roof construction, being covered, 
of course, with standard roof coverings to protect them from the 
weather. While certain stock sizes are recognized, it is customary 
to make the tile to order, so that they will fit into the supporting 
members of the roof. 

(2) GYPSUM PLASTER BOARD 

Gypsum plaster board is a board made of calcined gypsum 
which is used as a lath-and-plaster construction. It consists, 
essentially, of a thin layer of gypsum contained between two 
layers of paper. Plaster boards are 32 by 36 inches and vary 
in thickness from one-fourth to one-half of an inch. The gypsum 
contains wood fiber as an aggregate, which reduces its brittleness, 


I 


Gypsum — Properties , Definitions, and Uses ‘ 13 

so that the boards can be nailed directly to the studs. The 
surfaces of the boards are roughened, and the paper is not sized. 
This construction forms a good bond for the application of plaster. 

(3) GYPSUM WALL BOARD 

Gypsum wall board is quite similar in construction to plaster 
board. Wall board, however, is not intended to be plastered, but 
the surface of the board forms the finished wall. Therefore the 
surface of a wall board is smooth, and the paper is sized. Wall 
board comes in strips 32 or 48 inches wide, varying in length from 
4 to 10 feet. Its thickness may be from three-eighths to five- 
eighths of an inch. It is erected on walls in such a way that all 
joints are vertical, the length of the board being cut to correspond 
with the height of the ceiling, thereby avoiding horizontal joints. 
The boards are nailed directly to the studs or ceiling supports. 
The joints may be covered with wooden strips, which give the 
wall a paneled effect. Some types of wall board have the edges so 
finished as to permit of the joints being filled with a putty of 
calcined gypsum. Another method is to cover the joints with 
strips of paper the same color as that forming the surface of the 
board. Either of these methods produces a joint which is barely 
visible. 

(b) STRUCTURAL MEMBERS CAST IN PLACE 

Gypsum structural members, either plain or reinforced, can be 
cast in place in a manner similar to that used for pouring concrete. 
For this purpose, certain peculiarities of the material must be 
borne in mind. The one underlying principle is that maximum 
strength can be attained by getting as much gypsum as possible 
into a given space. A calcined gypsum to which nothing but re¬ 
tarder has been added should be used. Sand is purely an adulter¬ 
ant, the use of which may or may not be justified by'the reduced 
cost. An excess of water is particularly bad; use as little as pos¬ 
sible and still have the material workable. The time of set of 
calcined gypsum can be regulated at will by adding retarder or 
accelerator in the mixer or mortar box. Gypsum will set without 
drying out. Its maximum strength is attained when it is set and 
dry, and is not dependent upon its age. Intelligent use of these 
three properties enables the contractor to regulate the time of set 
so that he can remove the forms at his own convenience, and so 
that each member will be strong enough to take its full load before 
the superstructure need be erected on it. 


14 Circular of the Bureau of Standards 

9. PACKAGES AND STORAGE 

Crushed raw gypsum is usually shipped in bulk. The ground 
material for land plaster comes in cotton or burlap bags holding 
ioo pounds each. 

The best grades of pure calcined gypsum are shipped in paper- 
lined wooden barrels, which hold from 250 to 320 pounds net. 
Neat gypsum plasters come in burlap bags holding 100 pounds or 
in paper bags holding 80 pounds each. Ready-mixed (sanded) 
plasters are shipped in burlap bags of 100 pounds. 

Calcined gypsum will absorb water; otherwise it would not set 
and would be of no value. Therefore, when this material must be 
stored, it is necessary to protect it from the weather. It should 
even be protected from dampness if it is to be stored for six 
months or longer. When reasonably protected from moisture, it 
will keep indefinitely and may be considered as being good until 
used. Cases are on record where calcined gypsum stored in tin 
cans or in paper-lined wooden boxes showed no deterioration in 10 
years. 

10. WORK OF THE BUREAU OF STANDARDS 

The work which this Bureau is conducting on the subject of 
gypsum may be divided into three general classes: Cooperation, 
research, and routine. 

The Gypsum Industries Association, with offices in Chicago, is a 
national organization of manufacturers of gypsum and gypsum 
products. It maintains a fellowship at the Bureau of Standards 
to assist in conducting that research work which is necessary to 
advance the interests of the industry. 

The Bureau of Standards’ Plastering Conference is endeavoring 
to write specifications covering the use of gypsum plaster. 

At the request of the Gypsum Industries Association or of the 
American Society for Testing Materials, or frequently on its own 
initiative, the Bureau has undertaken research work on gypsum. 
Methods for measuring the time of set, normal consistency, fine¬ 
ness, strength, and similar properties of calcined gypsum have 
been developed and adopted. A method has been invented for 
making colored plaster from gypsum wood-fibered plaster. The 
acoustics and the fire resistance of plastered walls and the properties 
and uses of gypsum tiles are subjects now under investigation. 

Routine tests are conducted as a basis of purchase when gypsum 
products are bought by other branches of the Government service. 


i5 


Gypsum — Properties, Definitions, and Uses 

The American Society for Testing Materials is developing speci¬ 
fications for gypsum and gypsum products. The Bureau of Stand¬ 
ards and the Gypsum Industries Association are largely respon¬ 
sible for the work on this subject which has been accomplished by 
this society. 

Specifications for calcined gypsum, neat gypsum plaster, plaster 
board, and wall board have been adopted as tentative by the Amer¬ 
ican Society for Testing Materials, and are recommended by the 
Bureau of Standards as given in sections n, 12, 13, and 14, which 
follow. 

The data upon which these specifications are based have been 
developed by the Bureau of Standards with the full cooperation of 
the Gypsum Industries Association. 

11. SPECIFICATION FOR CALCINED GYPSUM 

1 . Definition. —Calcined gypsum is the product resulting from 

the partial dehydration of gypsum by means of heat. * 

2. Testing. —The chemical and physical properties of calcined 
gypsum shall be determined in accordance with the Tentative 
Methods for Tests of Gypsum and Gypsum Products (serial desig¬ 
nation, C 26-20 T) of the American Society for Testing Materials. 

I. CLASSES AND SIZES 

3. Classes. —Calcined gypsum is divided into three classes on 
the basis of its purity, as follows: 

Class A. —Containing not less than 88.4 per cent of 
CaS 0 4 • p2H 2 0. (These figures correspond to 90 per cent purity in 
the gypsum.) 

Class A A. —Containing not less than. 71.7 per cent of 
CaS 0 4 .jTH 2 0 , nor more than the similar quantities specified for 
Class A. (These figures correspond to 75 per cent purity in the 
gypsum.) 

Class AAA. —Containing not less than 60.5 per cent of 
CaSO^XlTO, nor more than the similar quantities specified for 
Class AA. (These figures correspond to 64.5 per cent purity in 
the gypsum.) 

4. Sizes. —Calcined gypsum (all classes) may be sold in one of 
the following sizes: 

No. 1 .—Material of this size shall all pass a 14-mesh sieve, and 
not less than 75 per cent of it shall pass a 100-mesh sieve. 

No. 2. —Material of this size shall all pass a 14-mesh sieve, and 
not less than 40 nor more than 75 per cent of it shall pass a 100- 
mesh sieve. 


16 Circular of the Bureau of Standards ' 

n. SAMPLING 

5 . Sampling. —At least 3 per cent of the packages shall be sampled 
and shall be so selected as to be representative of the contents of 
the shipment. Samples shall be taken from both the surface and 
the center of the packages. The material so obtained shall be 
thoroughly mixed and reduced by quartering to provide not less 
than a 15 pound (7.5 kg) sample for the laboratory. 

6. Laboratory Samples. —All laboratory samples shall im¬ 
mediately be placed in an air-tight container and shipped to the 
laboratory for test. 

in. CHEMICAL PROPERTIES 

7. The chemical composition, within the limits prescribed by 
section 3, shall be a matter of contract. 

IV. PHYSICAL PROPERTIES 

8. Fineness. —Calcined gypsum may be purchased in any of 
the sizes enumerated for calcined gypsum in section 4. Bach 
package or each shipment, shipped for resale, shall be accompanied 
by a tag or card which shall contain the information required by 
section 20. 

9. Color.—S pecification to be supplied by the committee when 
the information is available. 

10. Plasticity. —Specification to be supplied by the committee 
when the information is available 

1. Time oe Setting. —The time of set of calcined gypsum 
shall be suitable for the purposes intended, and shall conform to 
the provisions prescribed for such in the Tentative Specifications 
for Gypsum Plasters (serial designation, C 28-20 T) of the American 
Society for Testing Materials. 

12. Tensile Strength. —Calcined gypsum shall have a tensne 
strength of not less than 200 pounds per square inch (14 kg/cm 2 ). 

13. Compressive Strength. —Calcined gypsum shall have a 
compressive strength of not less than 1000 pounds per square 
inch (70 kg/cm 2 ). 

VI. PACKING AND MARKING 

20. Calcined gypsum may be shipped in either packages or in bulk. 

(a) When shipped for resale, the following information shall be 
legibly marked on each package or on a tag of suitable size attached 
thereto: 

Name of manufacturer; 

Brand; 

Description; 

Net weight of package. 


Gypsum — Properties, Definitions, and Uses 


17 


(b) When shipped in bulk, a card containing required informa¬ 
tion shall be conspicuously placed in the carrier. 

VII. INSPECTION AND REJECTION 

21. Inspection. —Inspection may be made either at the point 
of shipment or at the point of delivery. The inspector repre¬ 
senting the purchaser shall have free access to the carriers being 
loaded for shipment to the purchaser. He shall be afforded all 
reasonable facilities for inspection and sampling, which shall be 
so conducted as not to interfere unnecessarily with the loading 
of the carriers. 

22. Rejection. —Any rejection shall be based upon the specific 
cause of failure to conform to the requirements of these specifi¬ 
cations and shall be reported within 10 working days from the 
receipt of the shipment by the consignee. 

23. Rehearing. —Claims for rehearing shall be valid only if 
made within 20 working days from receipt of notice of specific 
cause for rejection. 

12. SPECIFICATION FOR NEAT GYPSUM PLASTER 

6. Definition. —Neat gypsum plaster is a plastering material 
in which not less than 85 per cent of the cementitious material is 
calcined gypsum, mixed at the mill with other materials. 

7. Composition. —Neat gypsum plaster shall contain not less 
than 85 per cent, by weight, of calcined gypsum. The remainder 
may consist of materials to control the working quality, setting 
time, and the fibering. 

8 . Time of Setting. — Neat gypsum plaster shall set in not less 
than i}/2 hours. 

9. Tensile Strength.—N eat gypsum plaster shall have a 
tensile strength of not less than 150 pounds per square inch (10^ 
kg/cm 2 ). 

13. SPECIFICATION FOR GYPSUM PLASTER BOARD 

39. Definition. —Gypsum plaster boards are used as a sheet 
lath or base for gypsum plaster on walls, ceilings, and partitions 
on the interior of buildings. 

40. Composition. —Gypsum plaster board shall consist of 
sheets or slabs composed of one or more layers of hydrated gypsum 
plaster, w'ith or without fiber, reinforced on the surface with 
chip board or felt. 


18 Circular of the Bureau of Standards 

41. Thickness. —The thickness of plaster boards shall average 

not less than the following: N 

(a) Three-eighths inch thick, with permissible local variations 
of one-sixteenth inch, plus or minus, and the thickness at any 
point in the board shall not be less than one-fourth inch. 

(b) Five-sixteenths inch thick, with permissible local variations 
of one-sixteenth inch plus or minus, and the thickness at any point 
in the board shall not be less than three-sixteenths inch. 

(c) One-fourth inch thick, with permissible local variations of 
one-sixteenth inch, plus or minus, and the thickness at any point 
in the board shall not be less than three-sixteenths inch. 

42. Dimensions.— (a) The width shall be 32 inches, with a per¬ 
missible variation of one-fonrth inch less than the dimension 
specified, and the length shall be 24, 36, or 48 inches, with a per¬ 
missible variation of one-half inch, plus or minus. 

(. b) Unless otherwise specifically stated in the order, plaster 
boards of the widths specified and in lengths of 18 and 30 inches 
may be included in amount not exceeding 5 per cent of any single 
carload. 

43. Weight. —The weight per thousand square feet of plaster 
board shall conform to the following: 

(a) For three-eighths inch thick, not less than 1500 nor more 
than 2000 pounds. 

(6) For five-sixteenths inch thick, not less than 1250 nor more 
than 1600 pounds. 

(c) For one-fourth inch thick, not less than 1200 nor more than 
1500 pounds. 

44. Strength. —(a) Strength-test samples shall be 12 inches 
wide and approximately 18 inches long, and when tested shall be 
supported on parallel knife-edge bearings spaced 16 inches and 
loaded through a similar bearing midway between the supports. 

(b) When tested as described, samples taken from the plaster 
boards shall carry not less than the following loads: 


Thickness in inches 

Ultimate load 

Load 
applied 
across 
fiber of 
surfac¬ 
ing 

Load 
applied 
parallel 
with fiber 
of sur¬ 
facing 

y s . 

Pounds 

40 

35 

30 

Pounds 

20 

17 

14 

ft. 

14 . 














19 


Gypsum — Properties, -Definitions, and Uses 

The minimum acceptable strength shall be not less than 5 
pounds below the averages given. 

Note. —The figures given above for five-sixteenths and one-fourth inch board have 
not yet been adopted by the A. S. T. M. 

(c) Samples tested shall fail by rupture of surfacing and core, 
and not by the breaking of the bond between the surfacing and 
the core. 

45. Cores. —The cores shall consist of the hydrated calcined 
gypsum plaster, to which may be added not to exceed 15 per 
cent by weight of sawdust or other vegetable fiber intimately 
mixed. Cores shall be of sufficient thickness throughout to make 
the finished plaster boards conform to the specifications as to 
thickness of the finished product. 

46. Surfacing Material. —The surfacing material shall be 
composed of plain chip board, felt, or other stock of the same 
character, which shall be securely bonded to the core and shall 
completely cover the larger surfaces, with a permissible variation 
of one-fourth inch at the edges along the shorter dimension. 

47. Finished Product. —The surfaces shall be such that they 
will readily receive and retain gypsum plaster. The edges and 
ends shall be reasonably straight and solid. The comers shall be 
square, with a permissible variation of one-fourth inch in the full 
width of the boards. The boards shall be free from cracks and 
imperfections that will render such boards unfit for use. 

Note. —Plaster boards will be considered acceptable that have their comers blurred 
or broken, provided that the broken portion is not more than inches in length. 

‘48. Packing and Marking. —Gypsum plaster boards shall be 
shipped so as to be kept dry and free from injury. Each board 
shall be plainly labeled with the name erf the brand and of the 
manufacturer. 

49. Inspection and Rejection. —Gypsum plaster boards shall 
conform to the foregoing requirements and shall be tested as pro¬ 
vided for in section 44 when determining their strength. Plaster 
boards ma,y be rejected upon failure to conform to any of the fore¬ 
going requirements. 

14. SPECIFICATION FOR GYPSUM WALL BOARD 

so. Definition. —Gypsum wall boards are used without plaster 
coatings as a finish on walls, ceilings, and partitions on the interior 
of buildings. 


20 Circular of the - Bureau of Standards 

51. Composition. —Gypsum wall board shall consist of sheets 
or slabs composed of a layer of hydrated gypsum plaster with or 
without fiber and a surfacing of chip or manila board on both sides. 

52. Thickness. —The thickness shall average not less than 
three-eighths inch, with permissible local variations of one- 
thirty-second of an inch, plus or minus, and the thickness at any 
point in the board shall not be less than five-sixteenths inch. 

53. Dimensions. —Where the wall boards are to be laid with 
joints butted, the width shall be 32, 36, or 48 inches, with a per¬ 
missible variation of three-thirty-seconds inch, plus or minus. 
Where the joints are to be filled with joint filler, the width shall 
b e 3 i M> 35M) or 47/4 inches, with a permissible variation of 
three-thirty-seconds inch, plus or minus. The length shall be 
4> 5> 6, 7, 8, 9, or 10 feet, with a permissible variation of three- 
eighths inch, plus or minus. 

54. Weight. —The weight shall be not less than 1500 nor more 
than 2000 pounds per 1000 square feet of wall board. 

55. Strength. —(a) Strength-test samples shall be 12 inches 
wide and approximately 18 inches long, and when tested shall be 
supported on parallel knife-edge bearings spaced 16 inches and 
loaded through a similar bearing midway between the supports. 

(6) Such samples taken from the wall boards shall carry a load 
of not less than 80 pounds when the line of supports is at right 
angles to the direction of the fiber of the surfacing, and not less 
than 32 pounds when the line of the supports is parallel to the 
fiber of the surfacing. 

(c) Samples tested shall fail by rupture of the surfacing and 
core and not by the breaking of the bond between the surfacing 
and the core. 

56* Cores. —The cores shall consist of hydrated calcined gypsum 
plaster, to which maybe added not to exceed 15 percent by weight 
of sawdust or other vegetable fiber intimately mixed. Cores shall 
be of sufficient thickness throughout to make the finished wall 
boards conform to the specifications as to thickness of the finished 
product. 

57. Surfacing Material. —The surfacing material shall be 
composed of plain chip, manila, filled news, or other stock of the 
same general character containing sufficient sizing to meet the 
following conditions: 

(a) Samples of the finished wall board shall sustain a static head 
of 1 inch of water (confined within a 2-inch ring on either of the 


Gypsum — Properties, Definitions, and Uses 21 

surfaces of the board) for a period of not less than two hours with¬ 
out penetrating the surface sufficiently to stain the core. 

(6) The surfacing material shall completely cover the two 
larger faces of the core and shall be securely bonded to it. 

58. Finished Product. —The surface designed to be exposed 
on erection shall be true and free from imperfections with or with¬ 
out decoration. The edges and ends shall be straight and solid. 
Where wall boards are to be butted the comers shall be square 
with both side edges. In cases where the joints are to be filled, 
the joints shall be square with both side edges with a permissible 
variation of one-eighth inch in the full width of the boards. The 
finished product shall be dry and free from cracks and imperfec¬ 
tions that would render such boards unfit for use. 

59. Packing and Marking. —Gypsum wall boards shall be 
shipped so as to be kept dry and free from injury. Each board 
shall be plainly labeled with the name of the brand of the manu¬ 
facturer. 

60. Inspection and Rejection. —Gypsum wall boards shall 
conform to the foregoing requirements and shall be tested as pro¬ 
vided for in sections 55 and 57 when determining their strength 
and water permeation. Wall boards may be rejected upon failure 
to conform to any of the foregoing requirements. 

Washington, September 18, 1920. 



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